all: remove travisci (#1680)

Remove `.travis.yml` and support for testing this repository on Travis CI.

Simplify the CI files we need to manage and maintain. @bartekn pointed out in #1542 (comment) that we don't use TravisCI anymore and we shouldn't keep it around and expect it to be kept up to date. For contributors who fork the repository and have TravisCI installed as an application in their GitHub account TravisCI also runs in addition to CircleCI because this file is present, which is distracting given it's not the build system we're relying on.

Author: leighmcculloch